1. Field of the Invention
The present invention relates to an improvement in a secondary electron spectrometer utilizing an electron beam probe.
2. Description of the Prior Art
The use of secondary electron spectrometers for undertaking voltage measurements at various locations of very small specimens, such as an integrated circuit chip, which utilize an electron probe is described, for example, in an article by the inventor herein entitled "VLSI Testing Using the Electron Probe," Scanning Electron Microscopy/1979, pages 285-296. The basic principle utilized in conducting voltage measurements at various circuit nodes utilizing an electron beam probe is that the voltage present at the circuit node causes the emission of secondary electrons, with the energy of the emitted secondary electrons being an indication of the voltage at the measurement point. The secondary electrons emitted at the sample pass through an extraction field and are subsequently decelerated in a homogenous opposing field. The result obtained from this conventional opposing field spectrometer is an integral energy distribution. The angle distribution of the secondary electrons is, however, not taken into consideration in such conventional devices. The angle distribution may, however, be changed due to electrostatic microfields at the surface of the specimen, that is, when the potential changes at the measurement point the local microfield at the specimen surface also changes, as does the angle distribution of the secondary electrons. Because secondary electron spectrometers of the type described in the above-identified article do not perceive the change in the angular distribution of the secondary electrons, measuring errors of approximately 5% through 10% occur.